1. Field of the Invention
The present invention relates to a technique for stepping down a voltage, and more concretely to a technique for stepping down a supply voltage so as to be supplied to an object circuit.
This application has foreign priority benefits of Japanese Patent Application No. 2007-336419 having a Japanese filing date of Dec. 27, 2007.
2. Description of Related Art
In case of each of processing systems provided with circuits that operate with a supply voltage that is lower than the system supply voltage, the system uses a step-down circuit to step down the system supply voltage to a supply voltage that operates those circuits. For example, in the fields of such semiconductor devices as the DRAM (Dynamic Random Access Memory and the pseudo SRAM (Static Random Access Memory), their elements are kept micronized more and more along with the progress of high density packing and highly integration techniques of LSIs. And accordingly, the internal operation supply voltage is lowered to improve the reliability of those micronized elements and reduce the amplitude of signal lines, thereby speeding up the operations and reduce the current consumption. On the other hand, when compared with those semiconductor devices, external devices such as processors, etc. are not micronized so much. When forming a processing system with use of such semiconductor devices, therefore, the system supply voltage is determined by the supply voltage of the processor, etc. Consequently, when forming a processing system that uses a single power supply, the system is designed to use a step-down circuit that steps down the system supply voltage to generate the required internal supply voltage.
FIG. 23 is an explanatory diagram showing an example of such a step-down circuit. In FIG. 23, a circuit group 10 includes plural circuits (circuit 1, circuit 2, . . . , circuit N) and those circuits are driven with a voltage (hereunder, to be referred to as an operation voltage) VINT0 that is lower than the system supply voltage VDD0. A step-down circuit 20 is driven according to a power supply enable signal VINTEN and the system supply voltage VDD0 is stepped down to the operation voltage VINT0 from the voltage VINT and supplied to the circuit group 10.
A circuit activation enable signal CE controls whether to activate/deactivate the circuit group 10. In the following description, an active period means a period during which an object circuit or circuit group is active and a standby period means a period other than the active period.
FIG. 24 is a diagram showing how the current consumption in the circuit group 10 changes, as well as how the voltage VINT and each signal supplied from the step-down circuit 20 make state changes while the circuit group 10 exits a standby period and enters an active period, then exits the active period and enters another active period.
As shown in FIG. 24, if the circuit activation enable signal CE is turned on while the circuit group 10 is in the standby period, the circuit group 10 is activated and enters the active period A1. Then, the power supply enable signal VINTEN is also turned on, thereby the step-down circuit 20 supplies the voltage VINT to the circuit group 10. At this time, the voltage VINT becomes the same as the operation voltage VINT0.
A curve L0 in FIG. 24 denotes how the voltage VINT changes in state. When the circuit group 10 is activated, beginning its current consumption. So, the voltage VINT goes lower than the operation voltage VINT0. Thus a so-called power supply drop occurs. The step-down circuit 20 then detects the power supply drop and raises the VINT up to the same level as that of the VINT0. However, it takes some time until the VINT corresponds to the current consumption of the circuit group 10. Hereunder, this period of time will be referred to as the step-down circuit response period A2.
If the circuit activation enable signal CE is turned off, the operation of the circuit group 10 ends and the circuit group 10 exits the active period A1 and enters the standby period. At this time, the current consumption of the circuit group 10 becomes 0. However, the step-down circuit 20 is required to supply charges continuously to the circuit group 10 so as to restore the power supply network and assure the stable capacity. Thus the power supply enable signal VINTEN is turned off later than the deactivated operation of the circuit group 10. This delay time is the restoration period A3 shown in FIG. 24. At the end of the restoration period A3, the voltage VINT is restored to the operation voltage VINT0.
Hereunder, this process will be examined from a viewpoint of the relationship between the charge supply capacity Qt of the step-down circuit 20 and the charge consumption Pt of the circuit group 10. In the period T1 that precedes the step-down circuit response period A2, the charge supply capacity Qt of the step-down circuit 20 is affected by the power supply drop and goes lower than the charge consumption Pt of the circuit group 10. Thus the voltage VINT begins falling step by step. And according to the restoration operation of the step-down circuit 20, in the period T2 that includes a succeeding step of the step-down circuit response period A2, the charge supply capacity Qt of the step-down circuit 20 comes to take approximately the same value as that of the current consumption Pt of the circuit group 10. As a result, the charge supply capacity Qt of the step-down circuit 20 exceeds the current consumption Pt of the circuit group 10 at the end point of the period T2, that is, at the start point of the period T3. Consequently, the power supply drop is eliminated.
The power supply drop functions a factor that causes troubles in the fast operation of circuits. Because the active period A1 becomes shorter when the circuit operation becomes faster while the step-down circuit response period A2 does not become short due to the power supply drop, the restoration period A3 is required to be long unavoidably. And because the voltage VINT is restored to the operation voltage VINT0 at the end point of the restoration period A3, the circuit group 10 cannot be activated during the restoration period A3. Thus the circuit group 10 cannot start the next operation. As the restoration period A3 becomes longer, it becomes difficult to keep the fast operation of the circuit group 10.
FIG. 25 shows an explanatory diagram of an internal power supply circuit 30 provided for plural memory cells and for a peripheral device. As shown in FIG. 25, the internal power supply circuit 30 includes a power supply terminal 31 for supplying the system supply voltage VDD0 and three step-down circuits 32. The internal power supply circuit 30 steps down the system supply voltage VDD0 and supplies the stepped-down voltage to an I/O interface 40, a peripheral logic circuit 50, and the plural memory cells 60.
The address and command buses consume large currents for writing/reading data to/from the memory. If the speed of the power supply responsibility is not insufficient, then the above-described power supply drop comes to become a significant one. Consequently, the sense-up driving performance falls, thereby it comes to take much time to amplify the memory cell signals.
Overdriving as described in, for example, the patent document 1 (Japanese Patent Application Laid Open No. 2000-57764) is one of the method for solving this problem. The overdriving method disclosed in the patent document 1 applies a voltage that is higher than the step-down circuit output to the sense-up PMOS side and a voltage that is lower than the ground potential to the sense-up NMOS side only when amplifying memory cell signals, thereby raising the transistor VSD to compensate the insufficient driving input.
FIG. 26 shows an explanatory diagram of a step-down circuit used for such overdriving. In FIG. 26, the same reference numerals are used for the same components as those in FIG. 23 for easier comparison between them.
In FIG. 26, the circuit group 10 is equivalent to the peripheral logic 50 or the group of the memory cells 60 in FIG. 25. OD denotes an overdrive signal. In a period in which this overdrive signal is turned on, the overdriving is carried out. Hereinafter, this period will be referred to as the overdriving period.
FIG. 27 is a timing chart for showing how the current consumption of the circuit group 10 changes, as well as the voltage VINT and each signal supplied from the step-down circuit 20 changes in state when the circuit group 10 exits a standby period and enters an active period, then exits the active period and enters another standby period after the overdriving is carried out as shown in the explanatory diagram in FIG. 26. The curve L1 in FIG. 27 denotes how the voltage VINT changes in such a case.
The overdrive signal OD is turned on synchronously with an activated operation of the circuit group 10. Then, overdriving is carried out. And when the overdriving period A4 reaches its end point, the overdrive signal OD is turned off, thereby the overdriving is deactivated. As shown in FIG. 27, when overdriving is carried out in such a way, the power supply drop value is reduced, the step-down circuit response period A2 is shortened, and the restoration period A3 is also shortened. In FIG. 27, the power supply drop is shown simply so as to be understood easily. The power supply drop can also be avoided completely by adjusting the voltage to be applied when carrying out overdriving. Such overdriving is effective to keep the fast speed of the circuit group 10.